The present invention relates to a graphical input terminal for generating an electrical signal representative of a specific location on an input or writing surface, with respect to a pair of coordinates, in which a character, figure or like graphical information is written by hand.
A graphical input terminal serves to detect a location on an input or writing surface in which graphical information has been handwritten with respect to a pair of coordinates and, then, generates an electrical signal representative of that two-dimensional location (coordinate signal). In a prior art graphical input terminal, two resistive sheets are arranged at a predetermined spacing from each other in such a manner that when information is handwritten in the surface of one of the resistive sheets, the two sheets are brought into contact in the handwritten point. This specific point on the input surface with respect to the coordinates is detected in terms of a change in the resistance between opposite sides of each resistive sheets.
A graphical input terminal of the type described is disclosed in U.S. Pat. No. 3,959,585. In the disclosed terminal, the spacing or air gap between a substrate and a sheet is designed small with a view to promoting convenient writing. However, the problem with such a terminal configuration is that after two resistive films have touched each other due to a pressure manually applied by a pen or like writing implement, a long period of time is consumed from the instant the pressure applied by the pen is removed to the instant the two resistive films bodily part from each other. That is, even after the pen has been removed from the interengaging point of the resistive sheets, the interengagement is maintained until air flows into that point via the surrounding air gap to sufficiently elevate the air pressure thereat.
As described above, the small air gap between the substrate and the sheet develops resistance due to the viscosity of air which results in a substantial period of time necessary for the air to be admitted in the interengaging point of the resistive films. Therefore, shifting the pen from one position to another on the writing surface produces a contact portion in the form of a line and this line remains over a certain period of time. It is not until the surrounding air flows into the linear contact portion to sufficiently elevate the air pressure thereat that the sheet regains its original flat configuration by surface tension, thereby allowing the two resistive sheets to separate from each other. During such a period of time, a current for detecting a two-dimensional position continuously flows through the linear contact portion, disenabling the information input point to be accurately detected.